Fix linker errors.

Build failure reported on kubuntu 14.04:
$ make
cc  -O2 -s -Wall -fPIE -pie  -ldl -o userhosts userhosts.c
/tmp/ccVLIqoI.o: In function `dlsym_or_abort.part.0':
userhosts.c:(.text.startup+0x5): undefined reference to `dlerror'
/tmp/ccVLIqoI.o: In function `init':
userhosts.c:(.text.startup+0x42): undefined reference to `dlerror'
userhosts.c:(.text.startup+0x55): undefined reference to `dlsym'
userhosts.c:(.text.startup+0x69): undefined reference to `dlerror'
userhosts.c:(.text.startup+0x7c): undefined reference to `dlsym'
collect2: error: ld returned 1 exit status
make: *** [userhosts] Error 1

Fix's rationale seems to be provide -ldl *after* symbol need becomes known,
so after source code is provided & built.

Makefile

@@ -8,7 +8,7 @@ PREFIX = /usr/local
 all: userhosts
 
 userhosts: userhosts.c
-	$(CC) $(CPPFLAGS) $(CFLAGS) $(LDFLAGS) $(LDLIBS) -o $@ $<
+	$(CC) $(CPPFLAGS) $(CFLAGS) $(LDFLAGS) -o $@ $< $(LDLIBS)
 
 install: all
 	install -Dp userhosts $(DESTDIR)$(PREFIX)/bin/userhosts